Datasheet LT1680 (Analog Devices) - 8

LT1680
U OPERATIO Basic Control Loop
supply will current limit and the output voltage will fall out of regulation. The average current limit circuit monitors The LT1680 uses a constant frequency, current mode the sense amplifier output without slope compensation or architecture. The timing of the IC is provided through an ripple current contributions. Therefore, the average input internal oscillator circuit that can be synchronized to an current limit threshold is unaffected by duty cycle. external clock and is programmable to operate at frequen- cies up to 200kHz. The oscillator creates a modified
Undervoltage Lockout
sawtooth wave at its timing node (CT) with a slow charge, rapid discharge characteristic. The LT1680 employs an undervoltage lockout circuit (UVLO) that monitors the 12V During typical boost converter operation, the MOSFET IN supply rail. This circuit disables the output drive capability of the LT1680 if the switch is enabled at the start of each oscillator cycle. The 12V supply drops below 9V. Unstable mode switching is switch stays enabled until the current through the switched prevented through 350mV of UVLO threshold hysteresis. inductor, sensed via the voltage across a series sense resistor (RSENSE), is sufficient to trip the current com-
Shutdown
parator (IC1) and reset the RS latch. When the switch is disabled, the inductor current is redirected to the supply The LT1680 can be put into low current shutdown by output. If the current comparator threshold is not reached pulling the RUN/SHDN pin low, disabling all circuit func- throughout the entire oscillator charge period, the RS tions. The shutdown threshold is a bandgap referred latch is bypassed and the main switch is disabled during voltage of 1.25V typical. Use of a precision threshold on the oscillator discharge time. This “minimum off time” the shutdown circuit enables use of this pin for under- protects the switch, and is typically about 1µs. voltage protection of the VIN supply and/or power supply sequencing. The current comparator trip threshold is set on the VC pin, which is the output of a transconductance amplifier, or
Soft Start
error amplifier (EA). The error amplifier integrates the The LT1680 incorporates a soft start function that oper- difference between a feedback voltage (on the VFB pin) and ates by slowly increasing current limit. This limit is an internal bandgap generated reference voltage of 1.25V, controlled by internally clamping the V forming a signal that represents required load current. If C pin to a low voltage that climbs with time as an external capacitor on the supplied current is insufficient for a given load, the the SS pin is charged with about 8µA. This forces a output will droop, thus reducing the feedback voltage. The graceful climb of output current source capability, and error amplifier responds by forcing current out of the VC thus a graceful increase in output voltage until steady- pin, increasing the current comparator threshold. Thus, state regulation is achieved. The soft start timing capaci- the circuit will servo until the provided current is equal to tor is clamped to ground during shutdown and during the required load and the average output voltage is at the undervoltage lockout, yielding a graceful output recovery value programmed by the feedback resistors. from either condition.
Input Average Current Limit 5V Internal Reference
The output of the sense amplifier is monitored by a single Power for the oscillator timing elements and most other pole integrator comprised of an external capacitor on the internal LT1680 circuits is derived from an internal 5V IAVG pin and an output impedance of approximately 50kΩ. reference, accessible at the 5V If this averaged value signal exceeds a level corresponding REF pin. This supply pin can be loaded with up to 10mA DC (20mA pulsed) for to 120mV across the external sense resistor, the current convenient biasing of local elements such as control comparator threshold is clamped and cannot continue to logic, etc. rise in response to the error amplifier. Thus, if average input current requirements exceed 120mV/RSENSE, the 8